right- the 2x5770's are going to perform pretty close to a 5870 in games that work properly with crossfire (most newer games will work pretty well), but could drop you to basically just 1x5770 for games that don't.
Personally, I would probably go for a single 5850 just because it gives you more power with less hassle, and it also gives you an upgrade path for the future if you feel the need for it (adding a second one later)
Edit- you probably don't need 750W for 2x5770 either- 2x5770 actually uses about the same amount of power as a 5870, or just a little more, so really you'll be fine with the same size PSU you'd get for a 5870. You'd be fine with a good 500-600W PSU.